This invention relates to cuvettes for use in photoanalysis apparatus and more particularly, to specially configured cuvettes for use in such apparatus.
Many different types of tests are performed by photoanalysis apparatus. For example, a test may be made to determine the light absorption of a fluid or the time rate of change of light absorption due to a chemical reaction. Typically, light is passed to a detector through a cuvette holding the fluid to be tested. The absorbence of the fluid in the cuvette is measured and used to determine certain characteristics of the measured fluid.
In photoanalysis systems performing the above described tests the cuvettes used to hold the test fluid must be of relatively high optical quality. The light must pass through the cuvette without substantial absorption, reflection or refraction. This requirement dictates that the cuvettes be manufactured from a relatively high quality material.
In order to further enhance the refractive characteristics of the cuvette, certain cuvettes have been manufactured in a square or rectangular configuration. When this type of cuvette is placed in a photoanalysis apparatus the light passes through the parallel side walls in a direction perpendicular to the walls themselves. This minimizes the distortion, reflection and refractions of the incident light beam as it passes through the cuvette while the substantially parallel rays incident unto the cuvette remain parallel after passage through the cuvette.
In some photoanalysis methods and apparatus, because of the small quantity of fluid available for analysis, and because of the light transmission characteristics in the tests performed, it is advantageous to pass a light beam through the cuvette and sample more than once. The problems associated with such a technique are of course apparent. Passing the light beam through a cuvette more than once will act to exacerbate any optical distortion, refraction or reflection of the light beam produced by the cuvette itself and will further act to increase the optical absorption resulting from passage of light through the cuvette.
The square or rectangular cross section of the cuvette would appear to provide the best light transmitting characteristics. However, if the cuvette is reused in subsequent tests, such as is true in an automated system, the square or rectangular cross section is an undesirable configuration. This is because the right angle corners in the cuvette act to trap portions of the fluid and hold the fluid so that it will be carried over to the subsequent sample contaminating the subsequent sample and possibly adversely affecting the test. Furthermore, a square or rectangular cuvette provides little or no possibility of controlling the beam width passed to the detector or the beam refraction.
The lowest possible carryover obtainable results when the internal cross section of the cuvette, which carries the fluid to be analyzed, is circular. However, to make an optimum optical absorption measurement, the light rays must go through the sample fairly parallel to each other and as previously noted, this dictates that the cross section of the cuvette be either rectangular or square. The problem presented then is to resolve this basic conflict between optimum absorption measurement and minimum carryover.
In addition to the desirability of a low carryover, the cuvettes used must be easy to build and low in cost, thus eliminating the possibility of forming exotic surfaces by expensive grinding and polishing operations.
Lastly, in some instances it is desirable to somewhat focus or control the light beam received by the detector, but this can only be accomplished while maintaining the light rays in a nearly parallel direction.